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The phosphorylation state of phosducin determines its ability to block transducin subunit interactions and inhibit
T Yoshida1, B M Willardson, J F Wilkins
1Physics Division, Los Alamos National Laboratory, University of California, New Mexico 87545.
Abstract:
Heterotrimeric GTP-binding proteins (G-proteins) serve many different signal transduction pathways. Phosducin, a 28-kDa phosphoprotein, is expressed in a variety of mammalian cell types and blocks activation of several classes of G-proteins. Phosphorylation of phosducin by cyclic AMP-dependent protein kinase prevents phosducin-mediated inhibition of G-protein GTPase activity (Bauer, P. H., Müller, S., Puzicha, M., Pippig, S., Obermaier, B., Helmreich, E. J. M., and Lohse, M. J. (1992) Nature 358, 73-76). In retinal rods, phosducin inhibits transducin (Gt) activation by binding its beta gamma subunits. While rod phosducin is phosphorylated in the dark and dephosphorylated after illumination (Lee, R.-H., Brown, B. M., and Lolley, R. N. (1984) Biochemistry 23, 1972-1977), the significance of these reactions is still unclear. The data presented here permit a more precise characterization of phosducin function and the consequences of its phosphorylation. Dephosphophosducin blocked binding of the Gt alpha 1 subunit to activated rhodopsin in the presence of stoichiometric amounts of Gt beta gamma, whereas phosphophosducin did not. Surprisingly, the binding affinity of phosphophosducin for Gt beta gamma was not significantly reduced compared with the binding affinity of dephosphophosducin. However, the association of phosducin with Gt beta gamma in a size exclusion column matrix was dependent on the phosphorylation state of phosducin. Moreover, the ability of phosducin to compete with Gt alpha for binding to Gt beta gamma was also dependent on the phosphorylation state of phosducin. No interaction was found between phosducin and Gt alpha. These data indicate that phosducin decreases rod responsiveness by binding to the beta gamma subunits of Gt and preventing their interaction with Gt alpha, thereby inhibiting Gt alpha activation by the activated receptor. Moreover, phosphorylation of phosducin blocks its ability to compete with Gt alpha for binding to Gt beta gamma.
Insights
Phosducin inhibits G-protein signaling in retinal rods by binding Gt beta gamma subunits. Phosphorylation of phosducin blocks this inhibition, impacting G-protein GTPase activity and signal transduction.
Area of Science:
- Molecular Biology
- Cellular Signaling
- Neuroscience
Background:
- Heterotrimeric G-proteins (G-proteins) are crucial for signal transduction.
- Phosducin, a phosphoprotein, inhibits G-protein activation.
- Rod phosducin's phosphorylation state in response to light is known but its functional significance is unclear.
Purpose of the Study:
- To precisely characterize phosducin function and the impact of its phosphorylation.
- To elucidate the mechanism by which phosducin modulates G-protein activity in retinal rods.
Main Methods:
- Size exclusion chromatography to assess phosducin-Gt beta gamma interactions.
- Binding assays to determine affinity of phosphorylated and dephosphorylated phosducin for Gt beta gamma.
- Experiments to evaluate phosducin's ability to compete with Gt alpha for Gt beta gamma binding.
Main Results:
- Dephosphophosducin inhibited Gt alpha 1 binding to activated rhodopsin, while phosphophosducin did not.
- Phosducin's association with Gt beta gamma and its ability to compete with Gt alpha for Gt beta gamma binding were phosphorylation-dependent.
- No direct interaction was observed between phosducin and Gt alpha.
Conclusions:
- Phosducin inhibits Gt alpha activation by binding Gt beta gamma subunits, preventing Gt alpha interaction.
- Phosphorylation of phosducin abrogates its inhibitory function by preventing competition with Gt alpha for Gt beta gamma binding.
- These findings clarify phosducin's role in regulating G-protein signaling in retinal rods.